TheFork users who ate in Waterloo have said really good things about Le Boucher du Maximus, La popote, cantine locale et bio and La Pâte et Ose. Maybe you should give them a try!
Some of the more popular Good for families restaurants having a terrace in Waterloo according to TheFork users include:
Comme à la Maison, with a 9.1 rating
Le Boucher du Maximus, with a 8.9 rating
Knokke Out Brussels, with a 8.4 rating